The inherent memory effect in ACTEL devices is responsible for the present high level of interest in the Mn doped ZnS ACTEL devices. Typically, the ACTEL device consists of a layer of ZnS:Mn film having a thickness of 0.5 .mu.m to 1.0 .mu.m that is sandwiched by a pair of dielectric layers of approximately the same total thickness as the ZnS. Various dielectric materials have been used such as amorphous BaTiO.sub.3. This structure is sandwiched between two conductors of which at least one is partially transparent.
An ACTEL device exhibits a brightness versus voltage amplitude hysteresis loop which is commonly referred to as a memory effect and as is shown in FIG. 1. The memory effect is characterized by a well-defined AC voltage threshold amplitude at which the luminescence begins and which reaches its maximum at V.sub.P. Once the voltage amplitude has been increased to a point where electroluminescence is obtained, the extinction of the luminescence occurs at a lower voltage amplitude. Between the extinction and turn-on voltage amplitudes, the device possesses a continuum of stable brightness states where the brightness of these states depends upon the voltage amplitude history.
This memory effect has been demonstrated with sine wave, square wave and pulse excitations where the pulses alternate in polarity. FIG. 2 is an example of a pulse mode of excitation. The pulse mode has the advantage of attaining a high brightness and it causes a low stress level in the device. However, a pulse mode of operation has the disadvantage of a fast memory decay.
FIG. 3 shows a square wave mode of operation which is also used in the prior art. The square wave mode has the advantage of a slow memory decay. It has the disadvantage of a lower brightness and higher stress characteristics on the ACTEL device.